The reduction in channel spacing within DWDM (dense wavelength division multiplexed) systems means that optical components used in the systems need to be characterized with ever-increasing wavelength resolution. One technique for characterizing an optical component provides stimulation to the component with a narrowband optical source, such as a tuneable laser. The response characteristics of the component to the stimulation are then measured with a broadband detector, such as an optical power meter. By tuning the optical source in fine wavelength increments, a characterization with a corresponding wavelength resolution is achieved over a wavelength range of interest. Wavelength resolution, however, is limited by the wavelength accuracy with which the fine wavelength increments can be tuned. In presently available tuneable lasers, wavelength inaccuracies can be large enough to make the tuneable lasers unsuitable for characterizing the optical components used in DWDM systems that have narrow channel spacing. In addition, these tuneable lasers have coherence lengths that are longer than needed to accurately characterize the optical components, making the measurement set-ups used in the characterization unnecessarily sensitive to optical reflections.
An optical source with improved wavelength accuracy and shorter coherence length described by Olivier Plomteux in the September 1999 supplement of Laser Focus World, Vol. 35, is well-suited for characterizing optical components used within DWDM systems. This optical source includes a fiber loop having an erbium doped fiber amplifier, a polarizer, a polarization controller, and a rotating dielectric filter, to provide high wavelength resolution over a wavelength tuning range of 1530-1570 nanometers. The polarized stimulus signal provided by this optical source relies on maintaining designated polarization states at each optical wavelength within the fiber loop so that conditions sufficient for oscillation are met over the wavelength tuning range.. Because polarization states of the components within the fiber loop are effected by aging, fiber movement, and other factors, periodic calibration is required to maintain the designated polarization states and compensate for the factors that effect polarization. Periodic calibration ensures that conditions for oscillation are met and that accurate wavelength tuning of the optical signal is achieved.
There is a need for an optical source that does not rely on maintaining designated polarization states to achieve oscillation, and that has sufficiently high wavelength resolution and short enough coherence length to stimulate optical components, such as those included in DWDM systems, so the components can be accurately characterized.
In an optical source constructed according to the preferred embodiment of the present invention, optical signals are generated as a result of varying polarization states within an optical loop to achieve oscillation. The optical signals have narrow spectral width and the optical source is tuneable, so that optical components stimulated by the optical signals can be characterized over a predefined wavelength range with high wavelength resolution. The optical loop includes an optical gain element, a tuneable filter and a polarization scrambler that provides a varying polarization transfer function. To attain oscillation within the optical loop, and thereby generate the optical signals, the optical gain element has sufficiently high gain within the passband of the tuneable filter and the polarization transfer function is sufficiently varied. Variation in relative delays between principle polarization states introduced by the polarization scrambler ensure that the condition for oscillation within the optical loop are met, even in the presence of factors that effect polarization. This varying polarization transfer function of the polarization scrambler produces a corresponding variation of the polarization of the generated optical signals, which are coupled from the loop to an output. Alternatively, the varying polarization transfer function is achieved by introducing wavelength-dependent polarization variations within the optical loop. When the polarization transfer function is varied in a random fashion, the polarization of the output signals becomes correspondingly randomized. A detector is optionally included with the optical source enabling transmission properties, reflection properties and other performance parameters of optical components to be characterized based on the detected response of the optical components to stimulation provided by the optical source at designated optical wavelengths.